Accelerometers have many different designs and are used for many different purposes. A monolithic accelerometer with signal conditioning, Model No. ADXL50 available from Analog Devices, Inc., of Norwood, Mass., however, has been successfully used in the automobile industry in connection with air bags. The Model ADXL50 is designed to have an output voltage directly proportional to acceleration. The manufacturers of the air bag systems design their air bag system to inflate the air bag when the output of the Model ADX150 reaches a certain voltage level. The specification for the Analog Devices ADXL50 is available from Analog Devices, Inc., One Technology Way, P.O. Box 9106, Norwood, Mass. 02062-9106.
Since the Model ADXL50 is actually implemented in automobiles, it must be able to withstand extreme temperature ranges and, because of its purpose, it must be highly reliable. Therefore, it is essential that each accelerometer shipped from the factory be tested. The ADXL50's specifications need to be guaranteed over certain temperature ranges (-40.degree. to 105.degree.), and power supply conditions (+4.75 volts to +5.25 volts) for the life of the accelerometer. The specifications are as follows: zero-g (DC bias without acceleration input) output voltage, sensitivity (output voltage to input acceleration scale factor), -3 dB bandwidth, linearity, output voltage high and low, noise, self-test input logic levels and input resistance, self-test output voltage swing, power supply rejection, quiescent supply current, and operating supply voltage. The zero-g output voltage and sensitivity parameters are key product characteristics when an accelerometer is used as part of an air bag system.
In the past, before the invention of this application, three different and physically separate test systems were used to test the accelerometers. In general, electrical tests were conducted on one semi-automated test bed. After the electrical testing was completed, the accelerometers were transported to a shaker test bed where the accelerometers were tested one at a time. The accelerometers were inserted into a fixture at the top of the shaker and a synthesizer was used to drive the shaker with a 7.5 g (amplitude), 100 Hz acceleration signal. A multimeter measured the RMS amplitude of the accelerometer output and a computer calculated the accelerometer's sensitivity by comparing the test device's output to that from the shaker's reference accelerometer. The shaker test bed only tested one specification, namely, sensitivity at +25.degree. C. and at 5 volts.
Another shaker test system was then used to mechanically test 400 devices at a time. The accelerometers were mounted on one of ten PCBs that hold up to forty devices. The loaded boards were inserted into a card cage mounted on the shaker. The top of the shaker was enclosed in a temperature chamber. Two different signals from each device (800 wires total) were routed out of the chamber to an external switch system (multiplexer). This system tested sensitivity over various temperature ranges; however, the test time was approximately eight hours for 400 accelerometers and the system was not capable of measuring the other accelerometer specification requirements.
The use of these three different and physically separate test systems raised a number of concerns. First, the test time was very slow. Second, the constant removal of the accelerometers and transporting them to the different test sites increased the likelihood of electrostatic discharge damage and lead integrity damage. Third, a special fixture was required for the electrical test bed and other special fixtures were required for the shaker test systems.
Most importantly, however, is the fact that there was no way to electrically test all of the accelerometers' specifications in a single test system under actual operating conditions, i.e., subject to acceleration forces.